In accordance with a known method for attaching an electronic component to a package substrate, a high-operating-temperature silicon semiconductor die is soldered to a copper heat spreader using a solder material whose melting or phase-transition temperature exceeds the nominal operating temperature of the resulting package. Since the coefficient of thermal expansion mismatch between the silicon die and the copper heat spreader is high, thermal stresses are induced in the solder during operation of the resulting package. Over time, thermal cycling is likely to generate cracks or voids in the solder, thereby reducing the area available for electrical and/or thermal transfer between the die and the heat spreader. The reduced heat transfer area, in turn, produces an increased junction temperature and may ultimately lead to a premature failure of the die. In some cases, the die can get separated from the heat spreader due to propagation of cracks in the solder.
In response, the prior art teaches the use of custom heat spreaders in an attempt to more closely match the thermal expansion coefficient of each heat spreader with that of the silicon die. For example, a laminated heat spreader is taught which includes a layer of material featuring a relatively low coefficient of thermal expansion, such as Invar, sandwiched between two layers of relatively higher thermal coefficient material, such as copper. However, such special copper-Invar-copper heat spreaders are costly to produce and, further, are less thermally conductive than their cheaper, monolithic copper counterparts. The die must, therefore, be operated at a lower power level to avoid undue thermal stresses and their attendant reliability problems.
Alternatively, the prior art teaches the use of relatively high temperature solder material to resist such crack propagation responsive to heat cycling. However, when such high-temperature solder materials are used, residual stresses are induced during solder reflow, which residual stresses contribute to premature failure of the assembly.